Revise optional dependency groups?

[wpbonelli]

Consider folding (some? all?) of the optional dependency groups into core requirements? There are 3 right now

• test: pytest, pytest plugins, various, for the test fixtures/etc
• dfn: tomli(-w), boltons, for the definition file functionality
• models: tomli(-w), boltons, pooch, filelock, for the models api

These are all fairly common developer-facing tasks and none of the dependencies are heavyweight.

I'm tempted to keep test and drop dfn and models, because test brings in more than the others

[wpbonelli]

As proposed in #266 (comment) we're tentatively thinking a test group and an ecosystem group.